The present invention relates to a polymer electrolyte fuel cell for use in portable power sources, electric vehicle power sources, domestic cogeneration systems or the like. More specifically, the present invention pertains to a conductive separator plate used therein.
A fuel cell comprising a polymer electrolyte membrane generates electric power and heat simultaneously by electrochemically reacting a fuel gas containing hydrogen with an oxidant gas containing oxygen such as air. The fuel cell comprises a polymer electrolyte membrane that selectively transports hydrogen ions and a pair of electrodes formed on both sides of the polymer electrolyte membrane, i.e., an anode and a cathode. The electrode is composed of a catalyst layer formed on each side of the polymer electrolyte membrane and a gas diffusion layer formed on the outer face of the catalyst layer. The catalyst layer is composed mainly of a carbon powder carrying a platinum group metal catalyst, and the gas diffusion layer has excellent gas permeability and electronic conductivity.
In order to prevent the supplied fuel gas and oxidant gas from leaking out or prevent these two kinds of gases from mixing together, gas sealing materials or gaskets are arranged around the electrodes so as to sandwich the polymer electrolyte membrane. The gas sealing materials or gaskets are combined integrally with the electrodes and the polymer electrolyte membrane beforehand. This is called an “MEA” (electrolyte membrane-electrode assembly).
Disposed outside the MEA are conductive separator plates for mechanically securing the MEA and for connecting adjacent MEAs electrically in series. The separator plates have a gas flow channel for supplying a reactive gas to the electrode surface and removing a generated gas and an excess gas. Although the gas flow channel may be provided independently of the separator plate, grooves are usually formed on the surface of the separator plate as the gas flow channel.
In order to supply the reactive gas to the grooves, it is necessary to use a piping jig which, depending on the number of the separator plates, branches off from the supply pipe of the reactive gas into the grooves of the separator plates. This jig is called a “manifold”, and the above-described type, connecting the supply pipe of the fuel gas from outside of the separator plates with the separator plates, is called an “external manifold”. A manifold having a simpler structure is called an “internal manifold”. The internal manifold comprises through holes that are formed in the separator plates with the gas flow channel. The through holes are connected to the inlet and outlet of the gas flow channel so that the reactive gas is supplied directly from these holes to the gas flow channel.
Since the fuel cell generates heat during operation, it is necessary to cool the fuel cell with cooling water or the like, in order to keep the cell under good temperature conditions. A cooling water flow channel is usually provided every one to three unit cells. Normally, the cooling water flow channel is often provided on the backside of the separator plate to form a cooling section. In a general structure of the fuel cell, the MEAs and the separator plates are alternately stacked to form a stack of 10 to 200 unit cells, a current collector plate and an insulating plate are attached to each end of the cell stack, and the resultant stack is sandwiched between end plates and clamped with clamping bolts from both ends.
The separator plates of such a polymer electrolyte fuel cell need to have high conductivity, high gas tightness, and high corrosion resistance to electrode reactions. Therefore, conventional separator plates have usually been formed from conductive carbon materials such as glassy carbon and expanded graphite, and the gas flow channel has been made by cutting or grinding the surface thereof or by molding in the case of expanded graphite.
Recently, separator plates that are compression molded of a mixture of graphite and resin for cost reduction are also in use. Further, an attempt is being made to injection mold separator plates out of a mixture of graphite and resin.
As the production method of separator plates, there is a proposal to injection mold separator plates by melting and kneading a compound of a mixture of graphite and a thermoplastic resin in an injection molding machine and injecting the compound into a mold (molding dies). However, since the separator plates need to have high electronic conductivity, the ratio of the conductive filler contained in the compound becomes high. In this case, the compound has high heat conductivity and low fluidity when melted, so that the moldability is significantly impaired, leading to such problems as insufficient filling of the compound and strength shortage of the welded parts. Another problem is that the performance of the fuel cell is deteriorated due to the limitations on the flow channel shape of the separator plate, the decrease in the strength of the manifold apertures arranged around the flow channel, the decline in gas tightness, etc.
In order to solve these problems associated with the conventional separator plates, it is an object of the present invention to provide a separator plate for a polymer electrolyte fuel cell having excellent conductivity and moldability. It is another object of the present invention to provide a polymer electrolyte fuel cell having excellent cell characteristics by using the separator plate for a polymer electrolyte fuel cell having excellent conductivity and moldability.